As conventional techniques related to power amplifier modules, there are JP-A-7-154169 and U.S. Pat. No. 5,629,648.
Very recently, while mobile communication markets typically known as cellular telephone systems are considerably expanded, new communication systems such as the broadband CDMA system and the EDGE system have been conducted. In such systems, high efficiencies and high linearity of portable terminal units are required. In particular, as to power amplifier modules which constitute one of major electronic components of terminal units, there is such a serious problem that these contradicting performance requirements are realized at the same time. Conventionally, many attempts have been made so as to solve this problem. As a typical example, in general, one control system has been proposed. In this system, an input power level of a power amplifier module is detected, and then, an operation condition of a post-stage amplifier which constitutes the power amplifier module is controlled by this signal. For instance, in a prior-investigated technical example shown in FIG. 3, a gate voltage of a transistor 111 which constitutes a post-stage amplifier 101 is controlled based upon a DC voltage obtained by envelope-detecting and smoothing an output signal of a pre-stage amplifier 102 in a DC voltage generating circuit 103 which is arranged by a directional coupling device 106, a detecting diode 107, and a low-pass filter 108. The DC voltage is increased/decreased in response to a power level inputted into a terminal 104, namely, a gate voltage of the post-stage amplifier 101 is controlled in response to the input power level. Also, in a prior-investigated technical example shown in FIG. 4, a power supply current of a first amplifier 201 is detected by a power supply voltage control circuit 203, and a power supply voltage corresponding to this current value is generated so as to control a power supply voltage of a second amplifier 202. Since the power supply current of the first amplifier 201 is changed in correspondence with a level of power inputted into a terminal 204, the power supply voltage of the second amplifier 202 is controlled in response to the input power level.
In the above-explained prior-investigated technical example, the DC voltage generating circuit 103 is required in order to envelope-detect and smooth the output power level of the pre-stage amplifier 102. However, since this DC voltage generating circuit 103 must be separately provided with reference to the amplifiers 101 and 102, there is such a problem that the stable characteristic of this DC voltage generating circuit 103 can be continuously and hardly guaranteed without any adjustment with respect to variations of environment conditions such as manufacturing deviation and ambient temperatures, and also power supply voltages. Furthermore, since the DC voltage generating circuit 103 is constituted by such components having different characteristics as the directional coupling device 106, the detecting diode 107, and the low-pass filter 108, there is another problem that the DC voltage generating circuit 103, and the amplifiers 101, 102 can be hardly formed in an integrated circuit.
In the prior-investigated technical example of FIG. 4, the DC-to-DC converter is required to be employed as the power supply voltage control circuit 203 for controlling the second amplifier 202. However, there are such problems that the use of such a DC-to-DC converter may constitute a factor of impeding the integrating manufacture of electronic components, and furthermore, the module is made bulky as well as the manufacturing cost is increased.
Also, the circuit system disclosed in the above-described U.S. Pat. No. 5,629,648 corresponding to the prior art may be considered. In this circuit system, since the output of the reference transistor, namely, the voltage at the bias point corresponding to the base input of the main transistor is not substantially changed in connection with the change in the input amplitudes, there is a problem that the output of the reference transistor can hardly give effectively influences to up/down of the operating point of the main transistor.